1. Field of the Invention
The present invention relates generally to a printer, and more particularly to a print head for actuating print wires with piezoelectric elements.
2. Description of the Prior Art
There has been proposed an actuator for a print head of a dot-matrix impact printer as disclosed in U.S. patent application Ser. No. 07/426,773 filed Oct. 26, 1989 or corresponding to European Patent Application Publication No. EP 0379780A2 published Aug. 1, 1990. The proposed actuator includes a piezoelectric element that is extendable and contractable in response to the application of a voltage, and a motion transmitting mechanism for amplifying extending and contracting movements of the piezoelectric element and transmitting the amplified movement to a print wire. When the amplified movements are transmitted to the print wires, the print wires make dot impressions on a print medium such as a sheet of paper to record desired information thereon in the form of a dot matrix.
In such printers, it is necessary to set the gap between the distal ends of the print wires and a platen (hereinafter referred to as "print gap") to a suitable dimension when the piezoelectric element is not energized. If the print gap were too wide, the impacting force applied by the print wires to produce dots would be too small, resulting in a low printing density. Conversely, if the print gap were too narrow, since the print wires move laterally along the platen while being projected toward and retracted away from the sheet of paper on the platen and an ink ribbon, the print wires would tend to stick into the paper or the ink ribbon while moving laterally even when the distal ends of the print wires slightly project from the tip end of the nose of the print head. If this happened, the print wires would be damaged or would catch the ink ribbon, with a resultant print failure.
To alleviate the above possible difficulties, it has been customary to adjust the gap between the distal ends of the print wires and the platen surface to a predetermined dimension when the print head is mounted on the printer. Alternatively, the print gap has been manually or automatically adjusted depending on the number of sheets of paper on the platen, the thickness of the paper on the platen, etc.
When the print wires become too short during long usage of the print head or an error occurs in a manual adjustment of the print gap, if the user continues printing without noticing the abnormal condition of a widened print gap, then the service life of the print head will be extremely reduced.